As stated above we need more info. If you are wanting a car that handles well then suspension and a torquee enigine setup is what you need. On the other hand if you want speed then you need to get more bhp out of your engine.
For handling you will want to add stiffer shocks and lowered springs, you might want to consider fitting an anti-roll bar and maybe welding up the rear beam. The engines generally give good torque, but if you are interested you could try fitting a 1600cc FIAT Tipo motor.
For speed you will need a new cam shaft, probably twin carbs.... the list goes on. So tell us what you want to use the car for and we'll be able to give advice.